Cancer is the second leading cause of death in the United States, after heart disease (Boring, C. C. et al., 1993, CA Cancer J. Clin. 43:7), and develops in one in three Americans, and one of every four Americans dies of cancer. Cancer is characterized primarily by an increase in the number of abnormal, or neoplastic, cells derived from a given normal tissue which proliferate to form a tumor mass, the invasion of adjacent tissues by these neoplastic tumor cells, and the generation of malignant cells which spread via the blood or lymphatic system to regional lymph nodes and to distant sites. The latter progression to malignancy is referred to as metastasis.
Cancer can be viewed as a breakdown in the communication between tumor cells and their environment, including their normal neighboring cells. Signals, both growth-stimulatory and growth-inhibitory, are routinely exchanged between cells within a tissue. Normally, cells do not divide in the absence of stimulatory signals, and, likewise, will cease dividing in the presence of inhibitory signals. In a cancerous, or neoplastic, state, a cell acquires the ability to "override" these signals and to proliferate under conditions in which normal cells would not grow.
Tumor cells must acquire a number of distinct aberrant traits to proliferate. Reflecting this requirement is the fact that the genomes of certain well-studied tumors carry several different independently altered genes, including activated oncogenes and inactivated tumor suppressor genes. Each of these genetic changes appears to be responsible for imparting some of the traits that, in aggregate, represent the full neoplastic phenotype (Land, H. et al., 1983, Science 222:771; Ruley, H. E., 1983, Nature 304:602; Hunter, T., 1991, Cell 64:249).
In addition to unhindered cell proliferation, cells must acquire several traits for tumor progression to occur. For example, early on in tumor progression, cells must evade the host immune system. Further, as tumor mass increases, the tumor must acquire vasculature to supply nourishment and remove metabolic waste. Additionally, cells must acquire an ability to invade adjacent tissue, and, ultimately, cells often acquire the capacity to metastasize to distant sites.
The biochemical basis for immune recognition of tumor cells is unclear. It is possible that the tumorigenicity of cells can increase when the cells' display of Class I histocompatability antigens is reduced (Schrier, P. I. et al., 1983, Nature 305:771), in that these antigens, in conjunction with tumor-specific antigens are required for the tumor cells to be recognized by cytotoxic T lymphocytes (CTLs). Tumor cells which have lost one or more genes encoding tumor-specific antigens seem to escape recognition by the corresponding reactive CTLs (Van der Bruggen, P. et al., 1991, Science 254:1643).
Once a tumor reaches more than about 1 mm in diameter, it can no longer rely on passive diffusion for nutrition and removal of metabolic waste. At this point, the tumor mass must make intimate contact with the circulatory system. Thus, cells within more advanced tumors secrete angiogenic factors which promote neovascularization, i.e., the growth of blood vessels from surrounding tissue into the tumor mass (Folkman, J. and Klagsburn, M., 1987, Science 235:442; Liotta, L. A. et al., 1991, Cell 64:327). Among these angiogenic factors are the fibroblast growth factor (FGF) and endothelial cell growth factor (ECGF). Neovascularization can, in fact, be an essential precursor to metastasis. First, the process is required for a large increase in tumor cell number, which in turn, allows the appearance of rare metastatic variants. Further, neovascularization provides a direct portal entry into the circulatory system which can be used by metastasizing cells.
A variety of biochemical factors have been associated with different phases of metastases. Cell surface receptors for collagen, glycoproteins such as laminin, or proteoglycans, facilitate tumor cell attachment, an important step in invasion and metastases. Attachment then triggers the release of degradative enzymes which facilitate the penetration of tumor cells through tissue barriers. Once the tumor cell has entered the target tissue, specific growth factors are required for further proliferation.
It is apparent that the complex process of tumor progression must involve multiple gene products. It is therefore important to define the role of specific genes involved in tumor progression, to identify those gene products involved in the tumor progression process and to further identify those gene products which can serve as therapeutic targets for the diagnosis, prevention and treatment of metastases of various forms of cancers.
Some attempts have been made to study genes which are thought to elicit or augment tumor progression phenotypes. Mutations may drive a wave of cellular multiplication associated with gradual increases in tumor size, disorganization and malignancy. For example, a mutation in the tumor suppressor gene which is a negative regulator of cellular proliferation, results in a loss of crucial control over tumor growth and progression. Differential expression of the following suppressor genes has been demonstrated in human cancers: the retinoblastoma gene, RB; the Wilms' tumor gene, WT1 (11p); the gene deleted in colon carcinoma, DCC (18q); the neurofibromatosis type 1 gene, NF1 (17q); and the gene involved in familial adenomatous polyposis coli, APC (5q) (Vogelstein, B. and Kinzler, K. W., 1993, Trends Genet. 9:138-141).
Insight into the complex events that lead from normal cellular growth to neoplasia, invasion and metastasis is crucial for the development of effective diagnostic and therapeutic strategies. The foregoing studies are aimed at defining the role of particular gene products presumed to be involved in tumor progression. However, such approaches cannot identify the full panoply of gene products that are involved in the cascade of steps in tumor progression. A great need, therefore, exists for the successful identification of those genes which are differentially expressed in cells involved in or predisposed to a tumor progression phenotype. Such differentially expressed gene and/or gene products can represent useful diagnostic markers and/or therapeutic targets for tumor progression disorders. With respect to diagnostic techniques, such genes and/or gene products could represent useful markers for the diagnosis, especially early diagnosis, given the correlation between early diagnosis and successful cancer treatment. With respect to therapeutic treatments, such differentially expressed genes and/or gene products could represent useful targets for therapeutic treatment of various forms of tumor progression disorders, including metastatic and non-metastatic neoplastic disorders, and for inhibiting the progression of pre-neoplastic lesions (e.g., hyperplastic lesions or other benign tumors) to malignant tumors.
Differentially expressed genes involved in tumor metastasis have been identified using murine melanoma cell lines of varying metastatic potentials, N-nitroso-methylurea-induced rat mammary carcinomas, mammary carcinoma cell lines, human breast tumors and spontaneous colonic and intestinal tumors in mice (Steeg, P. S., et al., 1988, J. Natl. Cancer Inst. 80:200-204; Qian, F., et al., 1994, Cell 77:335-347; Leone, A., et al., 1991, 65:25-35; Zou, Z., et al., 1994, Science 263:526-529; and Fodde, R., et al., 1994, Proc. Natl. Acad. Sci. USA 91:8969-8973).